1. Field of the Invention
The present invention relates to an improved method for recovering of organic acids, and in particular degradation-sensitive organic acids, from a solution.
2. Background of the Invention
It is often necessary to recover organic acids from solution. To this end, solutions containing organic acids can originate from a variety of chemical reactions and biological processes such as fermentation processes.
A typical approach to dealing with acid recovery involves the protonation of the carboxylate or salt form of the acid to reach a desired pH level.
For example, a process for the manufacture of syringic acid is taught in U.S. Pat. No. 4,191,841 in which an aqueous solution containing the double alkali salt of the acid is acidified with a strong acid such as hydrochloric or sulfuric acid followed by crystallizing the carboxylic acid product. The amount of strong acid addition is that sufficient to bring the solution pH to 3.
U.S. Pat. No. 4,202,828 describes a process to recover naphthoquinone and phthalic acid from a gas stream with an aqueous solvent formed by recycling the neutralized filtrate from naphthoquinone extraction and phthalic acid crystallization. The filtrate is treated with a base to a pH of 1.2 to 2.5, preferably 1.5 to 2.2, to neutralize only sulfuric acid and maleic acid byproducts without neutralizing unrecovered phthalic acid.
Acidification with hydrochloric acid to recover 3,6-dichloropicolinic acid from an aqueous solution of its salts in the process taught in U.S. Pat. No. 4,334,074. Once again, pH is used to determine the level of acidification.
Recovery of carboxylic acids, especially N-acetyl-DL-phenylalanine carboxylic acid, made via carbonylation using cobalt carbonyl catalyst is taught in U.S. Pat. No. 4,699,999. After removal of cobalt salt and any organic phase, the carboxylic acid is recovered from the aqueous phase by precipitating it with acids, especially mineral acids such as hydrochloric, sulfuric, or phosphoric acid, with the amount of acid addition based on a target pH of 1.
The recovery of N-methyliminodiacetic acid from an aqueous solution containing its disodium salt is described in U.S. Pat. No. 5,159,110. In this process, the solution is acidified using sulfuric acid and concentrated to precipitate sodium sulfate. Acidification is controlled by solution pH, with a target of approximately 2, the isoelectric point of the carboxylic acid.
Crystallization recovery of a dicarboxylic acid from an aqueous solution containing its disodium salt is also taught in U.S. Pat. No. 5,202,475. In this case the organic acid is 1,3- or 1,4-cyclohexanedicarboxylic acid, protonation is with hydrochloric or sulfuric acid, and the precise control of the acidification process via solution pH is again emphasized with preferred targets of 2.8 and 2.6 for the 1,4- and 1,3- isomers, respectively.
The recovery of hyodeoxycholic acid from swine bile is taught in U.S. Pat. No. 5,349,074. After isolation of the acid from other biliary acids and bile components as its magnesium salt, the solid hyodeoxycholic acid magnesium salt is suspended in an ethyl acetate aqueous solution and acidified with a mineral acid to produce the acid form. Solution pH is used to control acidification with a desired pH of 1 to 3.
The preparation of bis(amidocarboxylic acids), especially N,Nxe2x80x2-terephthaloyl-di(6-aminocaproic acid), under basic conditions is described in U.S. Pat. No. 5,410,076 in which the product is recovered by selective precipitation. Sulfuric acid was used to lower the pH and precipitate the organic acid with a pH of between about 5.5 and 6.5 disclosed as providing a highest purity.
An enzymatic method for producing L-aspartic acid from maleic acid is taught in U.S. Pat. No. 5,741,681 in which the reactions occur with the acids in their ammonium salt forms. Acidification to an isoelectric point of L-aspartic acid of 2.8 is disclosed as providing an improved recovery.
This reliance on pH for preformation is also prevalent in the context of organic acid recovery from fermentation broths.
For example, the production of lactic acid by fermentation followed by recovery utilizing extraction is described in U.S. Pat. No. 4,771,001. Prior to extraction the fermentation broth is acidified to a desired pH level.
The recovery of lactate esters and lactic acid from fermentation broth is described in U.S. Pat. No. 5,210,296 where the lactate salts are acidified in the presence of an alcohol of 4-5 carbon atoms. A strong acid such as sulfuric acid is added until the pH is between 1.0 and 1.6.
A process for recovering organic acids, especially lactic acid, from aqueous solutions such as fermentation broth is taught in U.S. Pat. No. 5,426,219 in which the solution is acidulated prior to extraction. Mineral acid is used to bring the pH between 1 and 4.5 before and during extraction.
Despite its widespread use, pH-based techniques have not proven to be consistently effective or even predictable.
For example, in those cases where the acid form of the organic compound is less soluble than the salt form, the desired carboxylic acid may be selectively precipitated by the addition of a stronger acid having a larger acid dissociation constant or smaller (pKa) than the desired product. In certain cases, increasing this acidification reduces the solubility of the organic acid and can improve recovery.
However, in other cases, the desired acid decomposes in the presence of a strong acid. In such cases, recovery may actually decrease upon acidification despite the reduced solubility of the acid. The presence of organic or inorganic impurities only serves to further complicate the recovery process. For example, such impurities may dissociate into cationic and anionic components, they can either compete for protons with the carboxylic acid or may donate protons into the process. Neither result is desirable.
That is, the competition for protons effectively reduces recovery of the desired organic acid while donating protons into the process can cause decomposition of the desired acid. Moreover, while the level of such impurities is typically small relative to the desired organic acid, such impurities can become concentrated during the process, thus, further magnifying their effect.
In light of the foregoing, it is clear that the existing protonation techniques are ineffective to consistently remove desired organic acids from aqueous solutions.
A second problem relates to the presence of anions other than that associated with the acid to be recovered. It is recognized that in the recovery of organic acids from an aqueous solution, the concentration of other anions present in the solution can strongly influence the effectiveness of the recovery process. Because of this, the art has looked towards removing the undesirable anions-as well as adjusting the solution pH-in attempting to improve the recovery process. The ion exchange technique has been employed but is equally ineffective.
In this regard, the carboxylate or salt form of the desired acid may be converted to the acid form by treating the aqueous solution with a hydrogen or free-acid form of a cation exchange resin. While this ion exchange technique may remove most, if not all, of the inorganic cations from the aqueous solution, it will also introduce its own set of problems.
For example, the resin will also protonate other anions in the solution. As was the case above, this can lead to detrimental effects. In particular, the resultant impurity acids, whether organic or inorganic in nature, are often stronger than the desired organic acid and thus may lead to decomposition of the desired acid. The ion exchange process may further dilute the solution leading to a still further decrease in recovery.
Accordingly, the need still exist for a process for which effectively recovers a desired organic acid, and in particular, a degradation-sensitive organic acid.
One organic acid of particular interest to the inventors is 2-keto-L-gulonic acid (KLG). To this end, techniques for the recovery of 2-keto-L-gulonic acid are also recognized in the art. However, existing techniques for the recovery of KLG are faced with a variety of problems in addition to the pH and ion-exchange problems discussed above.
For example, many of these techniques, such as those described in U.S. Pat. Nos. 2,421,611 and 2,421,612, teach crystallization recovery of this acid in its alkali or alkaline earth salt form followed by conversion to the acid form by treatment with an acid such as sulfuric. Such processes introduce a variety of problems insofar as yet another crystallization step is required to separate the 2-keto-L-gulonic acid from the aqueous acidification media for subsequent conversion steps such as esterification.
In addition, it is an art recognized belief that, in the production of a solid 2-keto-L-gulonic acid product, it is necessary to treat the aqueous solution with an anion exchange resin prior to recovery of the carboxylic acid using crystallization or extraction. See, for example, U.S. Pat. Nos. 4,990,441 and 5,202,476.
Thus, the need is particularly great for an improved process for the recovery of KLG from aqueous solutions such as fermentation broths.
The present invention is based, at least in part, on the surprising discovery that the optimum solution composition to recover desired organic acids cannot be effectively determined by pH measurements with or without the knowledge of the concentration of the organic acid in the solution.
Instead, the inventive process involves adjusting the proton level in the aqueous phase to a desired proton concentration where the proton concentration is selected, at least in part, relative to the amount of protons needed to associate with the organic acid to be recovered and the acid anions that are weaker than the organic acids to be recovered.
The present invention is based, at least in part, on the further surprising discovery that optimizing recovery of the desired acid does not require the removal of other anions from the solution. Accordingly, the inventive method can be effectively employed for separating a variety of acids from aqueous environments.
One aspect of the invention relates to a process for recovering a desired organic acid from a solution comprising the steps of:
(a) providing an aqueous solution including at least one desired organic acid or its acid anion;
(b) adjusting the proton concentration in the aqueous solution to a desired level, with the desired proton concentration being selected, at least in part, by the amount of available protons needed to associate with the acid anions of the desired organic acid(s) and/or acid anions that are weaker than the desired organic acids; and
(c) recovering at least a portion of the at least one desired organic acid from the aqueous phase.
In this regard, the desired proton concentration can be based on the amount of available protons being greater than, less than or substantially equal, to the amount of protons needed to associate with the anion of the desired organic acid(s) and acid anions that are weaker than the desired organic acid(s).
Specific examples of suitable organic acids include ascorbic, succinic, tartaric, glyconic; gulonic, citric, lactic, malic, maleic, acetic, formic, pyruvic, propionic, gluconic, butyric, itaconic acids and mixtures thereof.
One embodiment of the present invention relates to the recovery of 2-keto-L-gulonic acid (KLG) from aqueous solutions such as fermentation baths.